Several groups of lectins, i.e., carbohydrate-binding proteins, are known in man. One group is the C-type lectins. The C-type lectins contain a calcium-dependent carbohydrate recognition domain (a C-type CRD)1. Mannan-binding lectin (MBL), synonymous to mannose-binding lectin, mannan-binding protein or mannose-binding protein (MBP), belongs to the subgroup of C-type lectins, termed collectins, since these soluble proteins are composed of subunits presenting three CRDs attached to a collagenous stalk2. MBL interact with carbohydrates presented by a wide range of micro-organisms and accumulating evidence shows that it plays an important role in the innate immune defence3. When bound to carbohydrate MBL is able to activate the complement system.
The complement system may be activated via three different pathways: the classical pathway, the alternative pathway, and the newly described third pathway, the mannan-binding lectin (MBL) pathway which is initiated by the binding of MBL to carbohydrates presented by micro-organisms. The components of the alternative pathway and of the MBL pathway are parts of the innate immune defence, also termed the natural or the non-clonal, immune defence, while the classical pathway involves cooperation with antibodies of the specific immune defence4.
The human MBL protein is composed of up to 18 identical 32 kDa polypeptide chains27, each comprising a short N-terminal segment of 21 amino acids including three cysteine residues, followed by 7 repeats of the collagenous motif Gly-X—Y interrupted by a Gln residues followed by another 12 Gly-X—Y repeats. A small 34 residue ‘neck-region’ joins the C-terminal Ca2+-dependent lectin domain of 93 amino acids with the collagenous part of the molecule28.
The collagenous regions of the three polypeptide chains combine to form a subunit which is stabilised covalently by disulphide bridges. Individual subunits are joined by disulphide bridges as well as by non-covalently interactions27.
The position of these disulphide bridges has, however, not been fully resolved. SDS-PAGE analysis under non-reducing conditions of MBL shows bands with an apparent molecular weight (m.w.) larger than 200 kDa presumably representing blocks of 3, 4, 5 and even 6 assembled subunits27.
The actual number of subunits in the natural human MBL protein has been controversial. Lipscombe et al. (1995) obtained data by use of ultracentrifugation suggesting 25% of human serum MBL to be made of 2-3 subunits and only a minor fraction reaching the size of 6 subunits. The relative quantification was carried out by densitometry of Western blots developed by chemiluminescence27 found by SDS-PAGE analysis of fractions from ion exchange chromatography that the predominant species of covalently linked MBL subunit chains consisted of tetramers while only pentameric or hexameric complexes activated complement. Gel permeation chromatography (GPC) analysis, in contrast, suggests that MBL is comparable in size with the C1 complex. GPC can be carried out under conditions which allow for a study of the importance of weak protein-protein interactions in the formation of MBL molecules. MBL content in the GPC fractions can be determined by standard MBL assay techniques.
MBL is synthesized in the liver by hepatocytes and secreted into the blood. It binds to carbohydrate structures on bacteria, yeast, parasitic protozoa and viruses, and exhibits antibacterial activity through killing of the microorganisms by activation of the terminal, lytic complement components or through promotion of phagocytosis (opsonization). The sertiform structure of MBL is quite similar to the bouquet-like structure of C1q, the immunoglobulin-binding subcomponent of the first component in the classical pathway3. C1q is associated with two serine proteases, C1r and C1s, to form the C1 complex. Similarly, MBL is associated with two serine proteases MASP-15 and MASP-26, and an additional protein called Map 197. MASP-1 and MASP-2 have modular structures identical to those of C1r and C1s6. The binding of MBL to carbohydrates induces the activation of MASP-1 and MASP-2. MASP-2 then generates the C3 convertase, C4bC2b, through cleavage of C4 and C2. Reports suggest that MASP-1 may activate C3 directly. Nothing is known about the stoichiometry and activation sequence of the MBL/MASP complexes. MBL has also been characterized in other animals such as rodents, cattle, chicken and monkeys.
The concentration of MBL in human serum is largely genetically determined, but reportedly increases up to threefold during acute phase reactions8. Three mutations causing structural alterations and two mutations in the promotor region are associated with MBL deficiency9. MBL deficiency is associated with susceptibility to a variety of infections. Examination of five adult individuals with unusual and severe infections showed three to be homozygous for structural MBL mutations and two to be heterozygous10. Investigation of 229 children referred to the Danish National Hospital because of non-HIV-related immunodeficiency showed a tenfold higher frequency of homozygosity for structural MBL mutant alleles than seen in a control group11. Allotyping of 617 consecutively hospitalized children at St Mary's Hospital in London showed significantly higher frequency of homozygosity and heterozygosity for mutant allotypes in the infected children than in the noninfected12.
A wide range of oligosaccharides can bind to MBL. As the target sugars are not normally exposed on mammalian cell surfaces at high densities, MBL does not usually recognize self-determinants, but is particularly well suited to interactions with microbial cell surfaces presenting repetitive carbohydrate determinants. In vitro, yeast (Candida albicans and Cryptococcus neoformans), viruses (HIV-1, HIV-2, HSV-2, and various types of influenza A) and a number of bacteria have been shown to be recognized by MBL. In the case of some bacteria, the binding with MBL is impaired by the presence of a capsule13. However, even encapsulated bacteria (Neisseria meningitidis) can show strong binding of MBL14.
The microorganisms, which infect MBL deficient individuals, represent many different species of bacterial, viral and fungal origin12, 15-17. Deficiency is also associated with habitual abortions18. Indeed, MBL could be a general defence molecule against most bacteria, and thus be considered as one reason why so many bacteria are nonpathogenic.
While accumulating data support the notion of a protective effect of MBL there are also observations suggesting that infections with some microorganisms, notably intracellular pathogens, attain a higher frequency in MBL sufficient than in MBL deficient individuals19, 20. This is in concordance with the results of an animal experiment, where an increased number of HSV-2 were found in the liver of mice pre injected with human MBL21.
Clinical grade MBL has been obtained from blood donor plasma and shown to be safe upon infusion22. Production of recombinant MBL conceivably having a structure and an activity similar to that of native MBL has been attained (patent application PA 1999 00668/C5/KH).